Class D amplifiers convert an audio signal into high-frequency pulses that switch the output in accordance with the audio input signal. Some class D amplifier use pulse width modulators to generate a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors at a fixed frequency. Other class D amplifier rely upon pulse density modulators. Still other class D amplifiers may rely upon other types of pulse modulators. For heuristic purposes, the following discussion will only refer to pulse width modulators, but those skilled in the art will recognize that class D amplifiers may be configured with other types of modulators. The output of the class D amplifier is fed into a lowpass filter that converts the pulses back into an amplified audio signal that drives one or more audio speakers. This design approach produces an amplifier with better than 90% efficiency and that is more complex than its linear counterpart.
The class D amplifier requires an integrator, a duty-cycle modulator, a switch predrive circuit, and an output filter. The half-bridge class D amplifier using constant-frequency, duty-cycle modulation (FIG. 8), sums the square-wave output of the switching power transistors with the audio input to provide negative feedback. One cannot take the feedback after the lowpass filter unless one uses a complicated compensation network to handle the phase shift that the filter introduces. A two-pole filter, for example, would introduce a 180.degree. phase shift, which would cause the circuit to oscillate.
The square-wave output is synchronous with the audio input, but one must remove the carrier. The integrator sums the two signals and simulates the effect of the output filter. The circuit feeds the resultant error signal into the duty-cycle modulator, which comprises a comparator and a triangle-wave generator (FIGS. 9 and 10). Then, the circuit compares the triangle wave to the error signal to produce the modulated output.
The modulated output is a square wave whose duty cycle is proportional to the input signal. In the half-bridge circuit, this output drives the upper and lower power switches in antiphase; the circuit always drives one switch into saturation while it cuts the other off. The square wave causes the switches to change state as fast as possible, given the technology used to implement the switch. Fast switching limits the time that the switches spend in the linear operating region, thereby increasing efficiency and reducing heat generation. The combination of switching and conduction losses defines the upper bound of the amplifier's efficiency. The circuit filters out the high-frequency square wave that the power switches generate, leaving only the amplified audio signal. This signal then drives a ground-referenced speaker load.
Class D amplifiers are well known for their high efficiency which is typically greater than 90% at full power. But like class AB amplifiers, the efficiency of a class D amplifier is poor at low powers. This is especially true in full bandwidth class D amplifiers where the switching frequency is very high, 500 khz or greater. At low powers, the switching losses prevent class D amplifiers from achieving 90% efficiency. This is important in music applications which have a low average power with high peak powers. Thus, the efficiency benefits of class D amplifiers are rarely realized to their full potential in music applications. High power class D amplifiers radiate electromagnetic interference. This radiation can interfere with other electronics components. Such electromagnetic interference (EMI) is one ofthe main reasons that class D amplifiers have yet to be adopted as the industry standard. The output stage discussed below reduces EMI by minimizing both di/dt and dv/dt spikes. The result is a more efficient power amplifier that does not interfere with other electronic equipment.